HVAC systems in vehicles are designed to provide environmental comfort within a passenger space of the vehicle. The goal is to provide thermal comfort to passengers and acceptable cabin air quality. Vehicular HVAC systems typically employ separate heating and cooling systems. The heating system may use pump to move engine coolant to transfer heat from an engine to the passenger compartment. The cooling system may use a compressor to move refrigerant to transfer heat out of the passenger compartment. Energy from the vehicle is required to operate a pump or compressor, or other HVAC system component. Energy to operate HVAC system components may be supplied by a battery, a motor, or by an engine.
Fuel economy of an automobile is the relationship between the distance traveled and the amount of fuel consumed by the vehicle, typically expressed in terms of distance travelled per unit volume of fuel consumed (miles per gallon). In the case of a battery or hybrid vehicle, consumption may be expressed in terms of energy consumed per distance traveled (kilowatt-hour per 100 miles). Since the total force opposing the vehicle's motion (at constant speed) multiplied by the distance through which the vehicle travels represents the energy that the vehicle must expend in order to travel that distance, reduction of energy consumption should improve fuel economy/vehicle efficiency.
One factor potentially negatively affecting fuel economy/vehicle efficiency may be a short trip. Short trips may cause poorer vehicle efficiency ratings than longer trips. One reason may be that the passenger compartment may need extra energy to obtain a desired temperature or humidity faster for the short trip, whereas the longer trip may allow for less energy to be used per distance traveled to maintain an already existing cabin temperature or humidity. The energy expended during the pull-down/heat-up may be divided up over more distance providing for a higher efficiency rating.